1. Field of the Invention
The present invention relates to a cooling system for a motor, particularly to a technology of cooling a rotor of the motor.
2. Description of the Related Art
In a cooling system for a motor in a related art, since the temperature of a coil of a stator of the motor is closer to a heat-resistant limit than the temperature of magnets installed in a rotor of a motor, a cooling method for cooling the stator has been adopted. In the cooling method centering on the stator, oil is poured to an outer periphery of the stator to lower the temperature of the stator and the temperatures of the rotor and the stator of the motor are thereby controlled to a specified temperature or lower, for example.
However, in recent years, there are cases where the temperature of the rotor of the motor becomes closer to the heat-resistant limit than the temperature of the stator. To prevent the increase in the cost for rare earth element and export restrictions on rare earth elements, development for preventing the cost increase and for reducing production risk has been in progress. The development for preventing the cost increase and the production risk is to reduce the content of rare earth elements contained in the magnets used in the rotor of the motor, for example. However, the content of rare earth elements is reduced so that the magnets in the rotor of the motor is portions with the lowest heat resistance. Accordingly, it is now necessary to consider a method for directly cooling the rotor of the motor. In such a circumstance, for example, Japanese Patent Application No. 2011-83139 and Japanese Patent Application Publication No. 09-182374 (JP 09-182374 A) disclose cooling systems for a motor that include a cylindrical stator, a rotor, and a rotational shaft. In the cooling system oil supplied into a through hole formed along an axis in the rotational shaft to flow out through a cooling oil hole which passes through a peripheral wall of the rotational shaft to cool the rotor. The stator is fixed in a container-shaped casing. The rotor is positioned to have a prescribed gap from the stator in the stator. The rotational shaft is in a cylindrical shape which rotatably supports the rotor. In the above-described cooling system, as Japanese Patent Application No. 2011-83139 discloses, the oil supplied to the vertical through hole of the rotational shaft is also used as lubricating oil which is supplied to bearings rotatably supporting the rotational shaft, for example. In other words, in the above-described cooling system for a motor, the cooling oil passage for cooling the rotor communicates with the lubricating oil passage for lubricating the bearing.
In the above-described cooling system for a motor, an oil supply passage is designed so that the oil amount supplied to the rotor becomes optimal. However, for example, the oil supply ability varies depending on the rotation speed of the oil pump which supplies oil to the through hole of the rotational shaft, and the ability of discharging oil by the centrifugal force varies depending on the rotation speed of the rotor of the motor. Therefore, there may be a case where the oil supplied from the oil pump is not effectively used for cooling the rotor. Accordingly, excess oil which cannot be consumed for cooling the rotor is supplied to rotational bodies such as bearings and gears via the lubricating oil passage. This results in energy consumption for stirring the excess oil and in possible low rotation efficiency of the motor.